Permanent magnets based on rare earth-transition metal-boron (RE-TM-B) compositions containing iron and neodymium and/or praseodymium are now known and in commercial usage. Such permanent magnets contain as an essential magnetic phase grains of tetragonal crystals in which the proportions of iron, neodymium and boron (for example) are exemplified by the empirical formula Nd.sub.2 Fe.sub.14 B. These RE.sub.2 TM.sub.14 B magnet compositions and methods for making them are described in U.S. Pat. No. 4,802,931, assigned to the assignee of this application. The grains of the magnetic phase are surrounded by a second phase that is typically neodymium-rich as compared with the essential magnetic phase. It is known that magnets based on such compositions may be prepared by rapidly solidifying a melt of the composition to produce fine grained, magnetically isotropic platelets of ribbon-like fragments. Magnets may be formed from these isotropic particles by practices which are known and which will be discussed further herein.
Melt spinning is an efficient method of producing rapidly solidified particles of iron-neodymium-boron compositions. The melt-spun particles, either as is or after a suitable anneal, are magnetically isotropic and have high coercivity at room temperature. They may be used to make resin bonded magnets that are magnetically isotropic. The isotropic powder has many useful applications, but there is also a need for an anisotropic powder with a coercivity of at least 1,000 Oersted at room temperature.
It is also known that iron-neodymium-boron permanent magnets can be prepared starting with cast ingots or atomized powder of suitable compositions. The ingots or powder are comminuted to form micron-size (e.g., 1 to 15 microns) powder. These particles are magnetically anisotropic. They are aligned in a suitable magnetic field, compacted into magnet bodies and sintered to form permanent magnets.
When iron-neodymium-boron ingots are pulverized, the resulting powder is magnetically anisotropic, but it has little coercivity. Similarly, if a melt is atomized by conventional atomization techniques, such powder is magnetically anisotropic but has little coercivity. It is only after such powder has been compacted and sintered that the magnets display any appreciable coercivity. Workers have attempted to pulverize such anisotropic permanent magnets in order to obtain a coercive anisotropic permanently magnetic powder. Unfortunately, however, pulverization of the permanent magnet bodies yields a powder that has little coercivity.
It is known that rapidly quenched isotropic powder such as particles of melt spun ribbon can be suitably hot pressed and/or hot worked and plastically deformed to form high strength anisotropic permanent magnets. This practice is described in U.S. Pat. No. 4,792,367, assigned to the assignee of this application. Such magnets have excellent magnetic properties. U.S. Pat. No. 4,842,656, assigned to the assignee of this application, discloses how such rapidly solidified and further hot worked anisotropic alloy (unlike finely ground, oriented and sintered) can be comminuted, aligned in a magnetic field and bonded to make high coercivity anisotropic permanent magnets.
While excellent bonded magnets can be made from particles of magnetically anisotropic, hot pressed and/or hot worked alloy aligned in a magnetic field, it is the principal object of this invention to create like bonded magnets as well as fully dense hot pressed (i.e., binder-free) magnets from such magnetically anisotropic particles without application of a magnetic field in the particle consolidation step.